In a known manner, a turboshaft engine comprises an air inlet duct, a first air compression stage comprising a movable compressor wheel onto which the duct opens, a channel for conveying air compressed by the first compression stage to a second compression stage, a chamber for combusting a mixture of fuel and the air compressed by the compression stages and one or more stages for expanding the combustion gases.
In such a turboshaft engine, it is known to arrange seals between certain movable parts (rotor) and certain stationary parts (stator) of the turboshaft engine, in particular, on one hand, between the air inlet duct and the front of the first movable compressor wheel and, on the other hand, between the channel for conveying compressed air and the rear of the first movable compressor wheel. The terms “front” and “rear” are understood in relation to the direction of the axis of the turboshaft engine in which the flow of air flows overall in the turboshaft engine during operation.
It is thus known to arrange a sealing device comprising two seals between the front portion of the first movable compressor wheel and the air inlet duct. Said sealing device communicates, via the front seal, with a guide bearing of the rotor shaft of the turbine engine, which bearing is mounted in front of the device and comprises lubricating oil.
In order to keep the oil in the guide bearing in order to prevent it from leaking into the air inlet duct and causing the turboshaft engine to malfunction, air from the conveying channel, which is located downstream of the first compression stage, is conveyed to the sealing device so as to keep the two seals of the device under pressure. The terms “upstream” and “downstream” are understood in relation to the direction of the airflow.
More specifically, some of the airflow which is circulating in the conveying channel is diverted towards a second sealing device, which comprises a seal of which the function is to limit the flow rate of centrifugal air flowing along the rear face of the first movable compressor wheel. In fact, since the expanded airflow which flows along the rear face of the movable wheel then mixes with the airflow compressed by said wheel, too high a flow rate of air behind the wheel would reduce the efficiency of the compression.
Some of the airflow passing through the seal of the second device therefore flows along the rear face of the first movable wheel, whereas the remaining airflow flows along the rotor shaft to a cavity extending between the two seals of the first sealing device so as to keep said seals under pressure. The air in the cavity which keeps the two seals under pressure then flows both towards the guide bearing through the front seal of the first sealing device and towards the air inlet duct through the rear seal of the first sealing device. The airflow rate is therefore particularly reduced when the air is passing through the seal of the second sealing device, but generally allows the seals of the first sealing device to be kept under sufficient pressure so that the oil of the guide bearing is prevented from leaking into the air inlet duct of the turboshaft engine.
However, a problem arises when a reduction in pressure occurs in the air inlet duct upstream of the first movable compressor wheel which is caused, for example, by the presence of a grille, referred to as a pre-rotation grille, for guiding the airflow at the inlet of the movable wheel or by the presence of ice obstructing the duct during operation of the turboshaft engine in icy conditions.
Such a reduction in pressure in the air inlet duct leads to a reduction in pressure in the cavity which extends between the seals of the first sealing device, and this may cause the oil contained in the guide bearing to leak into the air inlet duct and therefore cause the turboshaft engine to malfunction, which is a significant drawback.